The present invention relates generally to a bake hardening cold-rolled steel sheet. The steel sheet has good baking hardenability, good dent resistance, and low yield ratio which contributes to the shape-fixability of the steel sheet. The present invention also relates a process for manufacturing the above steel sheet.
Recently, for the purpose of minimizing the fuel consumption, the thickness of the outer panel of a automobile has to be reduced. Thus, it is desirous of a high strength steel sheet capable of reducing the thickness of the outer panel of the automobile. Many kinds of high strength steels for this purpose, such as high strength low alloy steel, phosphorus added steel, dual-phase cold-rolled steel, and bake hardening steel have been suggested. However, the above-mentioned steels are unable to meet the requirements of this purpose.
The high strength low alloy steel is manufactured by adding a small amount of alloys into the matrix of the steel to increase the strength of the steel sheet and subsequently to reduce the required thickness of the steel sheet. However, the high strength low alloy steel is unable to be deformed without difficulty. In other words, the high strength low alloy steel has a poor shape fixability and thus, it is not suitable to be used as the outer panel of the automobile.
The rephosphorus steel is manufactured by adding phosphorus to the steel to elevate the drawability of the steel sheet. However, too much phosphorus will increase the yield strength of the steel sheet, and this will result in the increase of the spring back angle. This is, the shape-fixability of the steel sheet will get worse when too much phosphorus is added, and this will result in the increase of the flexibility of the steel. For this reason, rephosophrus steel is not suitable to be used as the outer panel of the automobile.
The dual-phase cold-rolled steel has a ferrite matrix with dispersed martensites therein, so that a good adjustment between the strength and the ductility of the steel sheet can be made, and the characteristics of high work hardening rate, low yield ratio, and continuous yielding can be obtained. However, the conventional dual-phase cold-rolled steel has poor baking hardenability.
The bake hardening steel having a ferrite matrix with cementites contained therein is manufactured by adding alloys and controlling the processes to obtain a steel containing a lot of carbon solid solutions which will contribute to the subsequent bake hardening of the steel sheet. However, a large amount of carbon solid solutions will easily cause yield point elongation during shape-forming of the steel sheet, and it will result in a poor outer appearance of the steel sheet. Thus, a large amount of cold-rolled temper extension is applied to eliminate the above-mentioned defects of the bake hardening steel. However, a large amount of solid solutions will cause room temperature ageing and cause the restoration of the yield point elongation.